In order to carry out drilling, and in particular drilling for oil, strings of drill rods are used which consist of drill rods assembled end to end in the course of drilling, which is effected with the aid of a drilling tool fixed at the end of the drill string and set in rotation around its longitudinal axis and subjected to a force in the axial direction due in particular to the weight of the drill string.
So as to ensure centring and guiding of the drill string in the interior of the borehole, it is known to use centring devices called stabilisers which have a central body of tubular shape and an external part having in particular at least one contact element by means of which the stabiliser comes into contact with the wall of the borehole in order to ensure the centring and guiding of the drill string.
The central body of the stabiliser has at its axial ends screw threads for connection to components forming the string of drill rods and generally to two drill rods between which the stabiliser is interposed as the string of drill rods is being assembled.
The external contact element of the stabiliser may be formed for example by blades in the axial direction or disposed helically around the external surface of the stabiliser.
Such stabilisers can ensure effective centring and guiding of the drill string in the interior of the borehole, but this is achieved by accepting permanent friction of the external contact element of the stabiliser on the wall of the borehole.
This friction increases the forces brought into play along the drill string during drilling and, in certain cases, may become excessive or lead to complete jamming of the drill string.
In the case of drilling in soft formations, the increase in the friction due to the presence of stabilisers may be reflected in a widening of the borehole, in particular in the diverted parts of the drilling where in the inclination relative to the vertical direction may be substantial.
The friction at the level of the stabiliser includes an axial component due to the displacement of the drill string in the axial direction of the drill string and a radial or circumferential component due to the rotation of the drill string in the interior of the borehole.
In order to carry out drilling in satisfactory conditions, it is necessary to limit or to manage as far as possible the coefficients of friction in rotation (μr) and in axial displacement (μa). Management of the coefficients μr and μa makes it possible to control the mechanical interactions between the elements of the string of drill rods and the wall of the borehole, in particular at the level of the stabilisers. This management makes it possible to optimise the centring and guiding of the drill string.
Stabilisers are known in which the external contact element of the stabiliser, which is formed for example by blades attached to or machined on the external part of the stabiliser or by a casing, is produced in such a way as to be rigidly joined to the central body or even integral with the tubular central body of the stabiliser.
In this case the contact element of the stabiliser which is rigidly jointed to the drill string is constantly in rotation during the drilling and in frictional contact with the wall of the borehole. The friction of the stabiliser in rotation in the borehole is reflected by a high radial friction μr.
Stabilisers are also known in which the external contact element of the stabiliser, produced for example in the form of a casing on which stabiliser blades or attached or machined, is mounted so as to rotate about the axis of the stabiliser and to slide in the axial direction on the tubular body or central shaft with a freedom of movement in the axial direction between two extremes in which the external contact element of the stabiliser comes into engagement with a means for coupling to the central shaft. Such a coupling means may consist for example of the teeth of a dog clutch system.
The external contact element of the stabiliser is retained in an intermediate position between its two end positions by resilient restoring means such as springs interposed between the external contact element of the stabiliser and the central shaft or body, at each end thereof.
Consequently, when the axial forces exerted on the contact element during drilling do not exceed a certain limit defined by the force of the springs, the contact element is free to rotate relative to the tubular central shaft or body. The tubular central shaft or body, which is joined to the drill string, can then turn in the interior of the external contact element of the stabiliser which is in contact with the wall of the borehole and which then only moves in axial translation.
In this type of operation of the stabiliser, the contact element of the stabiliser is immobilised in rotation against the wall of the borehole and the radial friction of the stabiliser in rotation, which is limited to the internal friction of a bearing for rotary mounting of the contact element on the tubular shaft, is very low.
When the axial forces exerted on the external contact element of the stabiliser exceed the limit fixed by the resilient restoring means, the contact element moves relative to the central shaft until a part of the end of the external contact element having a dog clutch toothing meshes with a corresponding toothing on the tubular central shaft of the stabiliser. The contact element of the stabiliser is then coupled in rotation to the tubular shaft and to the drill string, so that it is set in rotation on contact with the wall of the borehole.
The friction in rotation then increases considerably.
Such engagement of the contact element of the stabiliser in the rotation position with the drill string is produced in particular when the stabiliser encounters a narrowed or more or less closed zone of the borehole, either during drilling or during lifting of the tool. The axial forces applied to the contact element then increase and may exceed the limit of forces determined for the displacement of the contact element against the restoring springs. The setting in rotation of the stabiliser can ensure the boring or the re-boring of the borehole in order to permit the passage of the stabiliser and of the drill string, in particular when the stabiliser has blades having cutting edges.
When the axial force on the external contact element of the stabiliser returns to a level lower than the restoring force of the springs, the external contact element of the stabiliser returns to its intermediate position in which it is no longer driven in rotation by the central shaft and the drill string and is immobilised in rotation against the wall of the borehole. The friction in rotation of the stabiliser returns to a low level.
The axial friction during operation of the stabilisers, irrespective of whether they are of the first type, that is to say produced in one piece, or of the second type, that is to say with a contact element mounted so as to slide and rotate on the central shaft, remains very high.
Furthermore, in the case of stabilisers in which the contact element is mounted so as to rotate on the central shaft and is retained in a position which ensures the relative of rotation of the shaft with respect to an external contact element which remains immobile against the wall of the borehole, the elements for retaining the external contact element generally consist of springs interposed between the ends of the contact element and bearing surfaces of the central shaft. In this case, when the contact element is displaced in one direction, one of the restoring springs is compressed and the other spring is relaxed, such that the force applied by one of the springs increases while the force applied by the other spring decreases, so that perfect management of the control of the displacement of the contact element of the stabiliser is difficult.
When the contact element of the stabiliser is engaged in a coupling position with the central shaft by one of its end parts, the stability of this position is not ensured because the restoring forces are not exerted in an identical manner on the two ends of the external contact element.
U.S. Pat. No. 4,989,679 proposes, so as to improve the conditions for coupling of the external contact element of the stabiliser to the central body of the stabiliser in order to set it in rotation, to use a friction clutch which makes it possible to set the contact element in rotation progressively.
Such a device can only function correctly for moderate axial actuating forces. In particular, such stabilisers with a friction clutch cannot be used as stabilisers having a very wide operating range in so far as the axial forces are concerned.
Furthermore, the mounting of the contact element of the stabiliser between two restoring springs in the devices according to the prior art also limits the extent of the operating range in the non-rotary mode or also the threshold values of the axial forces of engagement of the means for driving the contact element in rotation.
This results in particular in risks of untimely engagement of the coupling means of the contact element for relatively low force values and an instability of operation of the stabiliser in non-rotary mode.
Therefore a stabiliser has not hitherto been known which makes it possible to ensure good management of the friction and in particular the axial friction during drilling in such a way as to limit this axial friction to a level permitting the drilling to be carried out in a continuous manner in good conditions.
In the case of stabilisers in one piece which are fixed in rotation to the drill string, the axial friction of the stabilisers is generally high and could not be modified by structural characteristics of these stabilisers in one piece.
In the case of stabilisers having a contact element mounted so as to be movable on a central shaft in order to pass from a non-rotary mode of operation to a rotary mode of operation, the management of the axial friction obtained by setting the contact element in rotation when the axial forces become excessive is not perfectly ensured due to the lack of reliability of the control of engagement of the contact element in order to set it in rotation.
Furthermore, such devices for management of the axial friction cannot be used for very high levels of force.
Finally, perfect stability of the contact element in the non-rotary mode of operation cannot be obtained within a wide range of axial forces in the interior of the borehole.